1. Field of the Invention
The present invention relates to a process for stabilization of heavy hydrocarbons by efficiently preventing sludge formation in storage tanks and/or transportation lines.
2. Description of Related Art
The composition of crude oils and their heavy hydrocarbon fractions varies greatly depending upon their geographic origins and types. Properties of several sample vacuum residues derived from various crude oils are shown in Table 1. As can be seen from Table 1, vacuum residues can have a sulfur content that ranges from 0.2 to 7.7 W % and a nitrogen content that ranges from 3800 to 7800 parts per million by weight (ppmw). Vacuum residues can also contain metals such as nickel and vanadium which make them difficult to process, since they deactivate or poison the catalysts used.
TABLE 1Properties of Sample Vacuum ResiduesSourceTachingBrentKirkukSafaniyaAthabascaBoscanRospomareSpecific Gravity0.9320.9841.0211.041.0381.0351.065API Gravity20.312.37.14.64.85.21.4Viscosity @ 100° F.1753808704000130040003500Sulfur W %0.21.575.235.424.945.567.66Nitrogen ppmw3800470040004300570078004200Conradson Carbon9.416.51824.616.719.326.3Residue (CCR)C5-Insolubles0.83.515.723.617.923.235.2C7-Insolubles0.317.713.610.214.123.9Nickel (Ni)1011524410112171Vanadium (V)7381251622801330278
In addition, the vacuum residues shown in Table 1 contain asphaltenes that can range from 0.3 to 35 W %, depending upon the source of the crude oil. Asphaltenes are defined as the particles precipitated by addition of a low-boiling paraffin solvent such as normal-pentane. They are solid in nature and comprise polynuclear aromatic hydrocarbons.
The chemistry of asphaltenes is complex. It is known that the asphaltene molecular composition differs from one asphaltene to another depending on the solvent type used, operating conditions and the oil source. It is also known that the amount of asphaltenes decreases with an increase in the carbon number of the solvent used to separate the asphaltenes, but with a loss in the quality of the treated oil. The asphaltenes recovered using high carbon number solvents are highly condensed structures and are likely to form sediment when there is a change of conditions, i.e., in processing or during storage.
The structure of the oil phase is well explained by Pfeiffer and Saal, who proposed a colloidal model of petroleum as schematically illustrated in FIG. 1. According to this model, asphaltenes are dispersed by resin molecules and small molecules such as aromatics that act as a solvent for the asphaltenes-resin dispersion; hydrocarbons are present as a non-solvent. If the oil composition is altered, i.e., by adding more hydrocarbon saturates or removing resins by means of reaction or physical separation, the equilibrium between the oil components changes, in which case asphaltenes start to flocculate out of the solution and can coalesce and precipitate.
Asphaltenes start to precipitate in oil storage tanks and/or transportation lines once they flocculate out of the solution. The accumulated precipitate of asphaltenes form a hard sediment, also referred to as “sludge.” The technical problems created by sludge formation include blockage of pipelines and burner nozzles, reduction in storage capacity, pump malfunctions, corrosion, false measurements and plugging. The factors controlling the sludge formation are oxidation, electrostatic charging, coagulation, volatility and the precipitation of wax and solid components, which usually result from changed conditions. Routine industrial maintenance of storage tanks unavoidably means the temporary inoperability of equipment. Furthermore, when conventional treatments are used to remove sludge, there is a potential for a significant negative environmental impact.
Solvent deasphalting is a process employed in oil refineries to extract valuable components from residual oil. The extracted components can be further processed in the refinery where they are cracked and converted into lighter fractions, such as gasoline and diesel. Suitable residual oil feedstocks which can be used in solvent deasphalting processes include, for example, atmospheric distillation bottoms, vacuum distillation bottoms, crude oil, topped crude oils, coal oil extract, shale oils, and oils recovered from tar sands. Solvent deasphalting processes are well known and described, for instance, in U.S. Pat. No. 3,968,023, U.S. Pat. No. 4,017,383 and U.S. Pat. No. 4,125,458, all of which disclosures are incorporated herein by reference.
In a typical solvent deasphalting process, a light hydrocarbon solvent, which can be a combination of one or more paraffinic compounds, is admixed with a residual oil feed to flocculate and separate the solids formed from the oil. Common solvents and their mixtures used in the deasphalting process include normal and/or iso-paraffins with carbon numbers ranging from 1 to 7, preferably from 3 to 7, including most preferably, propanes, normal and/or iso butanes, pentanes, hexanes, and heptanes. Under elevated temperatures and pressures, generally below the critical temperature of the solvent, the mixture is separated into two liquid streams, including (1) a substantially asphaltenes-free stream of deasphalted oil, and (2) a mixture of asphaltenes and solvent that includes some dissolved deasphalted oil.
While the solvent deasphalting process can be effective in removing almost all of the asphaltenes from the feedstock and thereby reduce sludge formation, a large portion of feedstock is rejected as asphalt due to the nature of the low carbon number paraffinic solvent used, resulting in a large loss in yield.
The problem addressed by the present invention is how to efficiently process heavy hydrocarbon feeds to prevent sludge formation in storage tanks and/or transportation lines while minimizing any adverse effects on the quality and yield losses of the hydrocarbon stream that is treated.